Pattern · Core

Composite

Build a tree of part-and-whole objects, then treat a single leaf and a whole branch THE SAME way through one interface.

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step 1 / 9
1/2Leaf and branch, one type
children*
«interface»
FileItem
+ size(): int
File
- bytes: int
+ size()
Folder
- children: List
+ size()
+ add(item)
Leaf and branch, one type
 1interface FileItem { int size(); }
 2class File   implements FileItem { ... }
 3class Folder implements FileItem {
 4    List<FileItem> children;   // files AND folders
 5}
✦

FileItem is the shared contract — anything in the tree can report its size().